[Ecls-list] Latest fixes

[Juan Jose Garcia-Ripoll]

* APPEND must not copy the last argument.

* MULTIPLE-VALUE-BIND is replaced with a LET form when it only defines
a single variable. This leads to faster and simpler code.

* The previous patches to make ECL work with ContextL broke
initialization of class slots, preventing re-initialization.

* In C:BUILDER, when the program is provided an :EPILOGUE argument,
this is executed with SAFE-EVAL, but without error value so that in
case of errors the debugger is entered. This problem was the cause
that SI:*BREAK-ENABLE* was set to NIL previously, preventing any use
of the debugger in standalone programs.

-- 
Instituto de Física Fundamental, CSIC
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